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Old 1st May 2011, 09:07   #1
MotionMan
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Default CV2 - How Does it Work?

I do not have a DCC system yet, or even a layout, so I'm not familiar with the setting up of CVs, but even so I decided I had to buy the recent product from Dapol, their N gauge Brittania Class.

My question is about the quality of the motion that DCC will be able to give this little locomotive, in particular I think my concern is with CV2, the start set up. The shop keeper was able to demonstrate the motion of the loco with a Lenz decoder in it, using a Lenz 100 controller, but at speed step 1 the loco still started with a slight jerk, obviously a lot better than with DC, but still an unrealistic start. I was told that the motion at speed one can be more finely adjusted by altering the voltage at speed one and he showed me a OO gauge loco that was using the same decoder as he had put in the N gauge loco (but a OO version) and he had experimented with CV2 and managed to obtain a very much more realistic start at speed one which was very impressive.

I left the shop happy and thinking I understood the process but then, on further reading, I got rather puzzled. What I'm puzzled about is the fact that the factory default setting for the voltage of CV2 appears to be zero and if you increase it you get a more sudden start at speed 1.

Therefore, if the higher the voltage the more sudden the start, how can you get a finer start at speed 1 if the factory setting is already at zero?

Is this something very difficult to describe in writing and best left for when I actually get my DCC system? I basically want some reasurance that I will be able to improve the start of this little locomotive with DCC.
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Old 1st May 2011, 09:30   #2
TCWORLD
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Default Re: CV2 - How Does it Work?

One possibility is that the jerky motion is the motor stalling - then it usually starts again at a high speed step when there is enough power to break the stall. If you increase CV2 then at the start it has enough power to get going from the off, preventing it from stalling (assuming it has good electrical pickup from the track of course!).

The other thing i have found useful in getting locos to start better is that some decoders allow you to control the PWM frequency that they use. Some frequencies work well with one motor but not another.

Thats my 2p worth on how it works.
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Old 1st May 2011, 13:42   #3
poliss
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Default Re: CV2 - How Does it Work?

Motors need more voltage to start than to keep moving. Altering CV2 will give the motor extra power, but only for a very short time, then it will settle to a lower voltage. See the Loy's Toys guide to CVs for a more detailed explanation of how it works. http://www.loystoys.com/info/configu...variables.html
Some basic decoders, such as the Hornby R8249, do not support CV2.
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Old 3rd May 2011, 19:37   #4
MotionMan
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Default Re: CV2 - How Does it Work?

TCWorld - thanks for your tuh-pence worth, but I'm not sure I understand what you mean about the motor stalling. In order to stall it has to have started. Realise I am referring to the actual start at speed step 1, not to jerky motion once it's started. Are you referring to something that happens in millionths of a second? Do you mean that in millionths of a second the motor imperceptably starts, stalls and then starts again at a higher voltage, and all the viewer sees is a sudden start at speed step 1?

Am I right that CV2 governs the speed at speed step 1? Or does it govern the way it reaches that speed i.e. straight to it suddenly or gradually.

In a DCC book I'm reading the author increased the starting voltage from 0% (factory default setting) to 20% and he states "For this particular model, the adjustment makes the locomotive start moving as soon as I turn the throttle". Now what does that mean? To me that sounds like an unrealistic sudden start. I think I'd have to see these starts demonstrated because people have different opinions on what is a good and bad start.

That's interesting about the PMW frequency. I'll look out for decoders with that facility.

Poliss - Are you stating that in order to get a smoother start you actually have to increase the voltage? Because the higher voltage is only given for a millionth of a second, and in pulses, and then the voltage that you've set for CV2 will drop once the loco is moving?

I've had a read of what it says on Loy'sToys web site and I understand the process a bit better now, thanks for that. It's full voltage all the time but in pulses, and it's the length of time of the positive pulses relative to the negative pulses that decides the speed (and direction) of the loco, but I still don't understand why the loco starts suddenly when the factory default setting of CV2 is zero voltage. How can it move at all if the voltage was set at zero? I must be missing something here.

I think what I need to know is the relationship between speed step 1 and CV2.
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Old 3rd May 2011, 20:08   #5
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Default Re: CV2 - How Does it Work?

What I should have said instead of, "then it will settle to a lower voltage", is "then it will send out fewer pulses". For example. Say a motor needs 1000 pulses of power to get going. But to keep going the motor only needs 100 pulses of power. The decoder will send 1000 pulses to the motor to get it started, but will then reduce the pulses to 100. This will happen so fast you won't be able to notice it. With CV2 set at zero without 'kick start' enabled, the decoder will only send out the 100 pulses, but without the 1000 pulses at the start.
You would have to go to a higher speed step to get the motor to start. That would be why it jerks.
The number of pulses I've quoted isn't accurate. It's just to give you an idea of what happens.
I think that's how it works anyway. :-) I could be wrong and if I am, no doubt someone will tell me.
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Old 4th May 2011, 00:13   #6
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Default Re: CV2 - How Does it Work?

What i imagine he means by starts on the first step, is that the loco starts moving at a crawl, or at least that is the aim. If CV2 is set too low, then the loco will not move, because the motor doesn't get enough voltage (and hence energy) to be able to start rotating and keep rotating.

Without going into too much detail of how a motor works, by stalling what I mean is it is unable to keep rotating. It will likely rotate 1/4 to 1/2 a cycle but then the magnetic field will collapse before it can get further, causing a stall. Due to the gearing you likely wont notice the loco move at all when this happens.

If you set the cv correct then the motor gets enough power to rotate complete cycles meaning the loco will move off slowly.

So in summary, if the CV is too low, it will stall and thus when it finally gets going at later speed steps it will jerk. If the CV is too high, it will suddenly lurk off the spot. If you tune it just right, it wont stall or jerk (hopefully )

Hope that clarified it.
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Old 9th May 2011, 08:38   #7
MotionMan
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Default Re: CV2 - How Does it Work?

Poliss - Yes, I understand what you mean, but I'm still confused about speed step 1. Using your example, if the motor needs 1000 pulses to get started, then if CV2 is set at zero and speed step 1 only sends out 100 pulses doesn't that mean the loco will not move at all at speed step 1? It would only move at whatever speed step sent out 1000 pulses, but this loco I bought did started moving at step 1, and apparently the factory default setting for CV2 is zero.

Remember, on this thread my only concern is with the transition from stationery to motion (and vice-versa) i.e. the movement from zero to speed step 1 (and vice-versa). To show you what I mean I'll link these two videos that I used on a previous thread. I hope they still work because I can't check on my home PC.

This is an example of a bad stop:-

http://www.youtube.com/watch?v=Tfp8J...feature=relmfu

Fast forward it untill the loco comes around again.

Now this is an example of how I want the starts and stops:-

http://www.youtube.com/watch?v=ygorZAGyk9w

I love this little video, and it's even the more amazing when you consider that this is N gauge (and the other video is O gauge).

At the moment the little loco I bought stops (and starts) like the O gauge loco in the first video (perhaps not quite as sudden). Being as the CV2 setting was apparently at zero will I be able to adjust it to get it to stop and start like the N gauge diesel in the second video? It has a five pole motor but no flywheel, and something called 'slow creep'.

TCWorld - thanks for the explanation. Quote:-

"What i imagine he means by starts on the first step, is that the loco starts moving at a crawl, or at least that is the aim."

Well, yes, I think so. Perhaps it depends what you define as a 'crawl'. I thought this little Brittania loco did start off at a crawl but the actual transition from stop to crawl was sudden. I was told that changing the number of speed steps from the default setting to 128 would not make the difference I am seeking. You see, the thing that's confusing me is that it did start at speed step 1, even though CV2 was at the default setting (zero). So it was getting enough pulses to start it at the first speed step, so if CV2 is already at zero how will giving CV2 a higher value make the motor start more smoothly? I think the key is that CV2 gives it a little nudge but just for a millionth of a second.

Also, I've just thought, does the CV2 setting also affect how it stops?

I'm getting confused. I think I'll just have to wait untill I go on a DCC fitting workshop later this month (if it has enough entrants to run) and actually do it. I just want the loco to start and stop like the N gauge loco in the second video, not like the O gauge loco in the first video. Have you had a loco that stopped and started like in video 1 and then by experimenting with CV2 you've got it to move more like video 2?
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Old 9th May 2011, 16:13   #8
poliss
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Default Re: CV2 - How Does it Work?

There are a few ways to find out how to fine tune decoders. You can do a lot of experimenting with various settings, find someone who's already done it, or you could buy a Sprog II. The Sprog II is a little box of electronics which is connected to your PC. It can be used by itself or can be plugged into your DCC controller. Once you've downloaded the free JMRI Decoder Pro program to your PC you can adjust CV settings very easily with on screen slider bars etc. The cost for the Sprog II with power supply is around 62.00.
Lots more information on the Sprog and JMRI websites http://www.sprog-dcc.co.uk/ http://jmri.sourceforge.net/

The loco in the first video uses MTHs own digital system, which is not compatible with NMRA DCC. Continental models usually have a better mechanism which enables them to run smoother, they also cost twice as much as British outline locos.

Nigel Cliffe has used Decoder Pro to program N scale locos. See http://www.2mm.org.uk/articles/CT-decoders.htm

What DCC controller are you thinking of getting btw? Remember that basic sets such as the Bachmann E-Z Command and Hornby Select can not program CVs.
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Old 11th May 2011, 08:25   #9
MotionMan
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Default Re: CV2 - How Does it Work?

Thanks very much for that Poliss, that looks very interesting, and all that lovely detail by Nigel Cliffe. I can't wait to get started but I haven't got a control set up yet (see below). The finer control I can get the better. The motion of the locos is very important to me. Psst, (whispers) there aren't any manufacturers reading are there? I'd rather pay twice as much for a loco and end up with one loco that moves realistically than have two locos that don't.

Quote:
Originally Posted by poliss View Post
What DCC controller are you thinking of getting btw? Remember that basic sets such as the Bachmann E-Z Command and Hornby Select can not program CVs.
Ah, now that is the big question. Which one do I get? I want the whole works


I was initially heading towards the ESU ECoS system with it's touch screen central station, but then I realised the advantages of that system can be much better supplied by getting just a hand held system and connecting to a PC or Laptop. I'm still not sure. This Saturday I'm getting the opportunity to try out the NCE system, which seems to fit the bill for me untill I heard that they haven't made a wireless version to UK specifications, which seems very odd to me. I wouldn't have thought it would be that difficult to change the frequency. I really want a wireless hand held unit. There's always something isn't there? You find the one you want and then .... there's one thing it doesn't do that you really want.
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Old 11th May 2011, 12:02   #10
poliss
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Default Re: CV2 - How Does it Work?

Ah, in that case take a look at the Roco/Fleischmann MultimausPro+Multizentrale. It's a hand held radio system that can be used in the UK. It also comes with Rocomotion PC software. It's cheaper than the ESU Ecos system too. The system itself costs 425.00. You'll need a suitable power supply too, such as the Lenz TR150 which is another 55.72. (Why don't manufacturers include power supplies with their DCC systems??) With a total cost of 480.72 it's still cheaper than the ESU Ecos which I found at 565.20.

The English manual which shows what it can do is at this link. http://www.roco.cc/fileadmin/downloa...pro/WEB_GB.pdf

Youtube video showing it in action here. http://www.youtube.com/watch?v=5PFe1sxSjTg
It works with several scales btw.
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